The known worlds can be best organized into a series of regions called "domains". The boundary between domains is indistinct, and mainly defines the difficulty in traveling between worlds. Two worlds easy to travel between would be in the same domain, with difficulty increasing the further they are apart.
Understanding the nature of domains can become even more confusing because the whole of a single world may not be within the same domain, or even consecutive domains. Kai-Chi represents a rare Flat Dimension which sits across two domains. This is more common in Finite Dimensions, where small regions of space can sometimes contain several domains. The area around the Utops Cluster, for example, sees at least three domains intersect within an area of 40 light years. There is also the famed lost world of Oberth, whose orbit supposedly took it through all seven known domains.
Domains can also overlap outside 3-dimensional space. Overlap points are especially easy to cross, and it is believed all forms of Inter-Dimensional Travel were discovered at these breach zones.
There are 7 known domains and an 8th suspected. The total number cannot be guessed at, but if it holds that the Multi-Verse is infinite, then so too is the number of Domains. There is also talk of a secret ninth Domain.
Individual domains are not named, only numbered 1-7. The official designations were first mapped and applied by the Gudersnipe Foundation during the Golden Age while constructing the GATE network. The existence of domains was previously known, but no systematic mapping existed.
The existence of a "secret domain" is based on an idiosyncrasy in the numbering system. The Foundation typically begins counts from 0, be it planets, hull numbers, or GATE hubs. That they would number the Domains 1-7 instead of 0-6 is considered highly suspect. According to sources within the Foundation, this is the result of an oversight early in the mapping project: the domains had originally been counted from 0, but after much work it was discovered the 0th and 1st domain were one and the same. Rather than re-number their entire system, they simply dropped the 0th domain and kept the others.
The existence of an 8th domain is only suspected, however it would be the most likely origin of Samuel Fate.